Why does a rocket blast off so fast?

The image of a rocket shooting off like a bullet is misleading. In the first seconds a large launcher moves slowly: thrust exceeds weight by only 20 to 40%, and net acceleration is a few meters per second squared, less than that of a sports car. What makes it seem like it's being fired off is that this acceleration keeps growing. The engine burns tons of propellant per second, mass drops, and the same thrust moves fewer and fewer kilograms. By the end of the first stage the load can reach 3 or 4 g, which is why some vehicles throttle down before separation.

Thrust depends on how much mass the engine expels and at what speed it leaves. A kerosene and liquid-oxygen engine ejects gas at about 3 km/s; a hydrogen engine, around 4.4 km/s. This exhaust velocity sets how much speed the rocket can gain per kilogram of propellant burned, and explains why carrying enormous tanks is tolerated: reaching orbit requires an effective velocity of about 9.4 km/s, adding the orbital speed itself plus losses from gravity and air drag.

The whole process is programmed. The vehicle passes through the point of maximum dynamic pressure at 60-90 seconds, often throttling down to protect the structure; afterward the atmosphere thins out and it can accelerate without resistance. Stages are jettisoned as soon as they're spent, so the final engine only pushes a fraction of the initial mass. This chain of events is what turns a slow start into an orbital velocity of 7.8 km/s in under ten minutes.